1. Field of the Invention
This invention relates to a roller board, more particularly to a roller board with a pivoting roller unit which is adapted to provide enhanced stability during turning movement.
2. Description of the Related Art
Referring to FIG. 1, a conventional roller board 1 is shown to include a horizontal rigid platform 10, first and second longitudinally spaced-apart base plates 21 disposed beneath and fixed to the rigid platform 10, first and second longitudinally spaced-apart wheel assemblies 30, 40 mounted on the platform 10 adjacent to the base plates 21 respectively, and first and second longitudinally spaced-apart roller units 2 mounted on the base plates 21 respectively.
With reference to FIG. 2, each of the base plates 21 has a cam retaining bore 211 and an accommodating chamber 210 which surrounds the cam retaining bore 211 and which spatially communicates with the cam retaining bore 211. Each of the roller units 2 includes a caster 22 mounted rotatably to the base plate 21 underneath the retaining bore 211, a roller 221 journalled to the caster 22, and a cam mechanism. The cam mechanism includes a cam 234 in the form of a rotatable shaft that is connected to the caster 22, and extends into the chamber 210 via the retaining bore 211 in the base plate 21, and that has an outer circumferential wall which defines a cam face 235 with a pair of diametrically opposite horizontal flat sides 237 and a pair of diametrically opposite curved sides 236 between the flat sides 237. A cam follower is disposed within the chamber 210 of the base plate 21, and includes an urging spring 24 that has a first end fixed to the base plate 21 and a second end which abuts against a selected one of the flat sides 237 for urging the cam 234 in a radial direction relative to the cam 234. Under this condition, the cam 234 is rotatable along with the caster 22 between a first position, in which, the second end of the urging spring 24 abuts against the cam face 235 at one of the flat sides 237 for urging the caster 22 to be maintained in a travelling direction parallel to a longitudinal axis of the base plate 21, as best shown in FIG. 3, and a second position, in which the second end of the urging spring 24 abuts against the cam face 235 at one of the curved sides 236 for urging the caster 22 to be maintained in a transverse direction relative to the longitudinal axis of the base plate 21, as best shown in FIG. 4. Each of the wheel assemblies 30,40 includes a pair of fixed wheels of a diameter that is smaller than that of the roller 221 to facilitate turning of the roller board 1 when the roller unit 2 rotates about its axis by virtue of swaying action of the rider""s weight to forward, rearward or lateral sides of the platform 10.
One disadvantage encountered during use of the aforesaid conventional roller board 1 resides in that during turning of the roller board 1 by virtue of swaying action of the rider""s weight, the second end of the urging spring 24 provides an abrupt change in orientation of the roller unit 2 upon reaching a juncture joined by an adjacent pair of the flat side 237 and the curved side 236. Such a sudden change in orientation of the roller unit 2 results in unstable stance of the rider on the roller board 1, and can cause the rider to fall off the roller board 1 in case the travelling speed of the roller board 1 is relatively high.
The object of this invention is to provide a roller board provided with a cam mechanism that is adapted to overcome the aforesaid disadvantage associated with the conventional roller board.
Accordingly, a roller board of the present invention includes a base plate, a roller unit, and a cam mechanism. The base plate has a cam retaining bore and an accommodating chamber which surrounds the cam retaining bore. The roller unit includes a caster mounted rotatably to the base plate underneath the cam retaining bore, and a roller journalled to the caster. The cam mechanism includes a cam in the form of a rotatable shaft that is connected to the caster and that extends into the accommodating chamber via the cam retaining bore of the base plate. The cam has a top end which defines an annular cam face with a pair of diametrically opposite lowermost points and a pair of diametrically opposite uppermost points between the lowermost points. A cam follower is disposed within the accommodating chamber of the base plate, and includes an urging spring that has a first end fixed to the base plate and a second end which is disposed above and which abuts against the cam face for urging the cam in an axial direction relative to the cam. The cam is rotatable along with the caster between a first position, in which, the second end of the urging spring abuts against the cam face at one of the lowermost points for urging the caster to be maintained in a travelling direction parallel to a longitudinal axis of the base plate, and a second position, in which, the second end of the urging spring abuts against the cam face at one of the uppermost points for urging the caster to be maintained in a transverse direction relative to the longitudinal axis of the base plate. Rotation of the cam between the first and second positions results in upward and downward movements of the second end of the urging spring in the axial direction.
In use the entire weight of the rider is concentrated on the urging spring, and movement of the second end of the urging spring between the first and second positions incurs a sufficient friction force, which in turn, ensures smooth and stable revolving of the roller unit relative to the base plate during swaying action of the rider for turning the roller board of the present invention.